The present invention describes an improved process for the rapid densification of high temperature materials including C—C composites, carbon and ceramic fiber reinforced preforms as well as carbon and ceramic foams.
Typically, these high temperature materials are densified using CVD/CVI (Chemical Vapor Deposition/Chemical Vapor Infiltration) of carbon and/or ceramic, or liquid infiltration with a resin and/or pitch as well as their combinations. The CVD/CVI process is highly capital intensive and suffers from long cycle times with multiple densification cycles typically taking several weeks to complete.
The impregnation of porous bodies with resins and pitches typically involves vacuum/pressure infiltration (VPI). In the VPI process a volume of resin or pitch is melted in one vessel while the porous preforms are contained in a second vessel under vacuum. The molten resin or pitch is transferred from vessel one into the porous preforms contained in the second vessel using a combination of vacuum and pressure. The VPI process is limited to using resin and pitches that possess low viscosity and associated low carbon yields. Therefore, densification of porous preforms with liquid resin and pitch precursors using the VPI process typically requires several cycles of impregnation followed by carbonization (frequently up to 7 cycles) and require long cycle times up to several weeks to achieve the desired final density.
To avoid the long cycle times associated with using low char-yield resins and pitches in typical VPI processes, high pressure impregnation/carbonization (PIC) is used to increase the carbon yield of pitches. Typical high-pressure carbonization cycles are in excess of 5000 psi and frequently 15000 psi. The resulting high char yield achieved with high-pressure carbonization allows the number of densification cycles to be reduced from 6–7 cycles to 3–4 cycles to achieve equivalent densities. However, the high-pressure vessels are capital intensive and of limited size thereby limiting the number of preforms densified in one vessel. The high pressures used also increase the risk of explosion and special safety precautions are required to meet safety standards.
An alternative approach to improve the efficiency of carbon densification processes involves the use of liquid resins with high carbon-yield (>80%). Typical high char-yield resins include synthetic mesophase pitches (e.g. AR mesophase pitch from Mitsubishi Gas Chemical Company, Inc., catalytically polymerized naphthalene) as well as thermally or chemically treated coal tar and petroleum derived pitches and other thermoplastic resins. However, there are many problems associated with using these high char yield resins in the current VPI processes related to their higher viscosity and associated higher process temperatures.
The present invention provides solutions to the above issues and provides a method to provide higher density composites with reduced cycle time. The present invention makes use of Resin Transfer Molding (RTM) technologies combined with high char yield resins to densify porous preforms within a matter of minutes.
RTM processes are not new. In recent years, resin transfer molding, or RTM, and its derivative processes (which are also called resin injection molding) have gained popularity in the aerospace, automotive, and military industries as a means of densification of porous preforms. In fact, RTM was originally introduced in the mid 1940s but met with little commercial success until the 1960s and 1970s, when it was used to produce commodity goods like bathtubs, computer keyboards and fertilizer hoppers.
RTM is typically used for the production of polymer-based composites. A fibrous preform or mat is placed into a mold matching the desired part geometry. Typically, a relatively low viscosity thermoset resin is injected at low temperature (100–300° F., 38–149° C.) using pressure or induced under vacuum, into the porous body contained within a mold. The resin is cured within the mold before being removed from the mold.
RTM has been shown to be uniquely capable of satisfying the low-cost and high volume (approximately 500–50,000) parts per year requirements of the automotive industry as well as the higher performance/lower volume (approximately 50–5,000) parts per year of the aerospace industry. Variations of the RTM process make it well suited for the production of large, complex thick-sectioned structures for infrastructure and military applications. An example of this is the lower hull of the Army Composite Armored Vehicle (CAV). The automotive industry has been using RTM for decades.
U.S. Pat. No. 5,770,127 describes a method for making a carbon or graphite reinforced composite. A rigid carbon foam preform is placed within a sealed flexible bag. A vacuum is created within the bag. Matrix resin is introduced into the bag through an inlet valve to impregnate the preform. The preform is then cured by heating. The resulting carbon or graphite structure is then removed from the bag.
U.S. Pat. No. 5,306,448 discloses a method for resin transfer molding which utilizes a reservoir. This reservoir comprises a pressure yielding porous sponge containing from about two to ten times the sponge's weight in resin. The resin reservoir facilitates resin transfer molding by providing a resin reservoir that can ensure the desired impregnation of a porous preform such as a porous fiber reinforced composite.
U.S. Pat. No. 5,654,059 discloses the fabrication of thick, three-dimensional mat structures comprising discontinuous thermoset pitch fiber, with needlepunch openings at least 80% through the structure.
U.S. Pat. No. 4,986,943 discloses a method for oxidation stabilization of pitch-based matrices for carbon-carbon composites. In this method, a lattice-work of carbon fibers is infiltrated with a pitch based matrix precursor, oxidized in an oxygen-containing atmosphere at a temperature below the pitch softening point, and carbonized to convert the matrix material into coke.
In typical extrusion processing of resins and plastics, a viscous melt is forced under pressure through a shaping dye in a continuous stream. The feedstock may enter the extrusion device in the molten state, but more commonly it consists of solid particles that must be subjected in the extruder to melting, mixing, and pressurization. The solid feed may be in the form of pellets, powder, beads, flake or reground material. The components may be premixed or fed separately through one or more feed ports.
Most extruders incorporate a single screw rotating in a horizontal cylindrical barrel with an entry port mounted over one end (feed end) and a shaping die mounted at the discharge end (metering end). A series of heaters can be located along the length of the barrel to separate the extruder into discrete heating zones. In typical extrusion applications a shaping die is used to form a fiber, rod or other shape. In RTM processes the shaping die can be replaced with a mold containing a porous body or preform.
Twin screw extruders are used less than single screw extruders, but they are widely employed for difficult compounding applications, devolatilization, and for extruding materials having high viscosity and limited heat stability. Twin screw designs can be either counterrotating or co-rotating, and the screw can be fully intermeshing, partially intermeshing or not intermeshing. Extrusion technology known in the art is discussed in Concise Encyclopedia of Polymer Science and Engineering, Jaqueline I. Kroschwitz, Ed., John Wiley & Sons, 1990, p. 363–367; and Principles and Plasticating Extrusion, Z. Tadmore and I. Klein, Van Nostrand Reinhold, New York, 1970.
Although the use of high char-yield resins provide the potential for improved carbon yield and reduced number of densification cycles required to achieve final density their use in VPI and RTM processes have been unsuccessful. Utilization of the high char yield resins in VPI processes has been restricted because the high char-yield resins have high viscosity and higher temperatures are required to lower the viscosity of the resin and pitch for impregnation. The higher processing temperatures and higher viscosity of the high char-yield resins lead to the following problems with existing VPI and RTM processes.
1) The resins begin to cure in the holding vessels prior to impregnation.
2) Higher pressures are required for impregnation of the high viscosity resin.
3) Non-uniform and incomplete infiltration of the resin into the porous body or preform leading to dry spots (porosity) caused by encapsulation of air pockets in the preforms.
The successful use of high char-yield resins in RTM processes would provide significant reductions in the densification cycle time of composite materials compared with existing CVD/CVI and VPI processes by reducing the number of impregnation cycles to achieve the required final density. In addition, the use of high char yield resins in RTM processes would also provide a reduction in resin waste (90% utilization of resin)
The successful use of high char yield resins in RTM processes requires several innovations including:    1) Means to provide efficient, uniform flow of the high viscosity resin into and throughout the preform.    2) Means to prevent the formation of dry pockets caused by a combination of incomplete impregnation of resin and entrapment of air and volatiles in the preform, and thereby maximize densification efficiency.
The prior art demonstrates the need for a method and apparatus for impregnating a porous preform with high viscosity molten resin (for example AR mesophase pitch) at high temperatures. The resulting impregnated preform is preferably free from “dry spots” and has the ability to undergo further processing such as oxidative stabilization, carbonization and graphitization.